1. Field of the Invention
The invention relates to a measuring device for detecting the humidity and temperature of a window surface facing a vehicle interior, particularly of a vehicle windshield including at least one capacitive moisture sensor and at least one temperature sensor, which are placed together on the window surface, and a downstream evaluation unit. Further, measuring signals from the moisture and temperature sensors are combined into a single signal, and affect the shape of the single signal.
2. Description of the Background Art
Heating and air-conditioning systems provide a high level of interior comfort to passengers in a motor vehicle. Further, during the automatic operation of an air-conditioning system, various exterior conditions can lead to window condensation. For example, window condensation can occur during an extended recirculation operation, at low or fast-dropping exterior temperatures, or when not enough fresh air is being supplied to the inside of the automobile. In these instances, the relative air humidity inside the vehicle rapidly rises, which is also contributed to by the passengers of the vehicle. Thus, sensor systems are used to detect the formation of window condensation either directly on the window or in the environment with suitable sensors.
For example, the German application No. DE 199 07 401 illustrates a method for detecting a relative humidity whereby the relative humidity is measured and transmitted to a control device of a ventilating unit. That is, a moisture-sensitive sensor is glued to the interior side of the windshield to thereby detect window condensation. In addition, at the onset of window condensation, and even before the condensation becomes visible, a capacitance of the electrical sensor abruptly changes and the abrupt change is evaluated and transmitted to a control device of the ventilation unit to initiate suitable measures such as introducing fresh air to prevent fogging or condensation on the windshield.
Further, the article in the magazine Automobile Technical Magazine 102 (2000) 1, at pages 42-44, illustrates a sensor used to determines a relative humidity using a capacitive thin-film sensor, and measures the temperature of the window surface at the same time. In addition, the sensor is a highly sensitive infrared radiation detector based on thin-film thermal elements, which allows for a contact-free measurement of the temperature. The evaluating unit then calculates the dew point temperature after converting analog input signals of the relative air humidity and the corresponding temperature, and uses the calculated dew point to determine the possibility of condensation forming on the windshield. The sensors are also mounted to the rear side of the interior mirror at a distance from the windshield.
In addition, German Application No. DE 101 52 999 illustrates a sensor for detecting a propensity of condensation forming on a windshield. In this application, a sensor module is arranged directly on the windowpane and is used to measure the moisture on the window as well as the temperature of the window surface. The measured signals are then transmitted to an evaluation unit, which calculates a propensity for condensation forming on the windshield.